pH to pOH Calculator – Easy pH and pOH Conversion

A pH to pOH calculator is used in chemistry to determine the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution.

In pure water at 25°C, the ion product constant (Kw) equals 1 × 10^-14, leading to the relationship pH + pOH = 14.

Let’s say we have a solution with a pH of 5.3:

• Input pH = 5.3 into the calculator.
• The calculator automatically applies the formula: pOH = 14 – pH.
• Therefore, pOH = 14 – 5.3 = 8.7.

pH to pOH Calculator

pH Value	pOH Value	Solution Type	[H+] (M)	[OH-] (M)
0.5	13.5	Strong Acid	3.16×10^-1	3.16×10^-14
2.5	11.5	Acidic	3.16×10^-3	3.16×10^-12
4.0	10.0	Acidic	1.00×10^-4	1.00×10^-10
5.0	9.0	Acidic	1.00×10^-5	1.00×10^-9
5.3	8.7	Acidic	5.01×10^-6	3.16×10^-9
6.0	8.0	Weakly Acidic	1.00×10^-6	1.00×10^-8
7.0	7.0	Neutral	1.00×10^-7	1.00×10^-7
8.0	6.0	Weakly Basic	1.00×10^-8	1.00×10^-6
9.0	5.0	Basic	1.00×10^-9	1.00×10^-5
9.5	4.5	Basic	3.16×10^-10	3.16×10^-5
10.2	3.8	Basic	6.31×10^-11	1.58×10^-4
11.0	3.0	Strong Base	1.00×10^-11	1.00×10^-3
12.0	2.0	Strong Base	1.00×10^-12	1.00×10^-2
12.5	1.5	Strong Base	3.16×10^-13	3.16×10^-2
13.0	1.0	**Very Strong Base	1 × 10^-14	1 × 10^-1

pH to pOH Conversion Formula

The conversion formula between pH and pOH is based on the ionization constant of water (Kw).

pH + pOH = 14 (at 25°C)

This equation can be rearranged to find either value:

• pOH = 14 – pH
• pH = 14 – pOH

These formulas are derived from the relationship:

• pH = -log[H+]
• pOH = -log[OH-]
• Kw = [H+][OH-] = 1 × 10^-14

For a solution with pH = 9.5:

pOH = 14 - 9.5

pOH = 4.5

How to Find pH and pOH?

To determine pH and pOH values:

• Measure directly using a pH meter.
• Calculate from [H+]: pH = -log[H+].
• Convert from pOH: pH = 14 – pOH.

• Calculate from [OH-]: pOH = -log[OH-].
• Convert from pH: pOH = 14 – pH.

Given [H+] = 3.2 × 10^-4 M:

pH = -log(3.2 × 10^-4)

pH = 3.49

Therefore, pOH = 14 – 3.49 = 10.51.

Why is pH-pOH always 14?

The constant sum of pH and pOH equaling 14 (at 25°C) is derived from the self-ionization of water. This fundamental chemical principle occurs when water molecules dissociate into equal numbers of H+ and OH- ions:

H2O ⇌ H+ + OH-

The equilibrium constant for this reaction (Kw) equals 1 × 10^-14 at 25°C. Taking the negative logarithm of both sides: -log(Kw) = -log([H+][OH-]) = 14

Since -log[H+] = pH and -log[OH-] = pOH, we get:

pH + pOH = 14

In pure water:

• [H+] = [OH-] = 1 × 10^-7 M
• pH = -log(1 × 10^-7) = 7
• pOH = -log(1 × 10^-7) = 7
• Verify: 7 + 7 = 14

What is pOH when pH is 8?

pH + pOH = 14.

For pH = 8:

pOH = 14 – pH

pOH = 14 – 8

pOH = 6

This means the solution is slightly basic, with an [OH-] concentration of 1 × 10^-6 M.

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